Here are some excellent sources, online and print, for further learning.

HTML: The Definitive Guide (O'Reilly and Associates, http://www.ora.com). The book's title is pretty accurate; this is as good a reference on HTML as you'll ever find.

The World Wide Web Consortium (W3C). If you really want the standard, this is the place to find it. The standard itself is rather formalized and cryptic to read, but when deciding if something is "correct" code or not, there is no substitute.

Fourth Corner Systems (http://www.4th.com). This tutorial is part of a larger web site that contains, among other things, The Proofreader's Companion, a quick reference to some common grammatical pitfalls.

HTML Goodies (http://www.htmlgoodies.com). This is a web site devoted to helping authors improve their sites. It has tutorials ranging from basic to advance, including Java, JavaScript, CGI programming, and other expert-level topics. There are also helpful code samples to illustrate the various techniques.

http://www.webpagesthatsuck.com. It's not a joke -- that's a real web site! The author is a professional web author and instructor, and he believes one of the best ways to learn good web design is by looking at some horrible examples from the real world, and then analyzing why they are bad.